There has been and continues to be a need to efficiently maximize the operational speed of brushless DC motors, and step motors in general. Brushless DC motors are multi-phased, typically 3 phased, motors that are commutated by the selective application of winding drive signals applied in reference to the motor shaft rotational position. Step motors and synchronous motors, when driven in a closed loop mode, behave in the same way as a brushless DC motor. For purposes here, brushless DC motors shall be considered to include step motors and synchronous motors. A basic understanding of such motor structures and operation is provided by Acarnley, "Stepping Motors: A Guide to Modern Theory and Practice", Peter Peregrinus Ltd., London, U.K., (1982/1984).
Brushless DC motors are often operated under closed loop control based on control signals generated by a motor shaft position transducer. Various sensor arrangements are known, including those based on Hall effect sensors, shaft mounted optical encoders, and other dedicated sensor/circuit systems.
Brushless DC motors generally exhibit a quasi-sinusoidal variation of torque with shaft position when one phase is excited with a constant current. Correspondingly, when the motor is driven by an external prime mover at a constant speed, and all the phases are open circuited, each phase generates a quasi-sinusoidal back EMF signal at a frequency and amplitude proportional to speed. The phase signal referred to in the previous sentence is understood to mean the voltage between the phase lead and the center tap in the case of a Wye wound motor, or the equivalent notional center tap for a Delta wound motor. Therefore, the supply voltage needed to sustain a desired speed increases in rough proportion with increasing speed. The back EMF is a proportion of the voltage that must be applied to the brushless DC motor to sustain a given shaft rotational speed.
In commutating brushless DC motors, the commutation waveforms of the power signals applied to the motor phase windings must be synchronized with the motor shaft position. A number of different general commutation waveforms have been used. Periodic rectangular waveforms are advantageous to produce a maximum top speed for the motor at a given supply voltage.
However, a disadvantage of using rectangular waveforms is that harmonic currents are created which heat the motor windings, but do not contribute to the production of torque. In addition, in many control systems, it is desirable to employ transconductance or high output impedance power amplifiers. By use of such drive amplifiers, the frequency response characteristics of the motor are relatively unaffected by temperature and motor impedance. However, the current waveform that results from a rectangular drive voltage typically has a peculiar shape due to the complexities of motor conductance and back EMF. Consequently, rectangular drive waveforms are difficult to apply in practice with high output impedance amplifiers.
Another method of achieving higher top speeds with a fixed supply voltage is to advance the phase of the drive signals as the required motor speed is increased. This method is described in Acarnley and Gibbons, "Closed Loop Control of Stepping Motors: Prediction and Realization of Optimum Switching Angle", Proceedings IEE (London), Vol. 129, Part B, No. 4, 1982, and also in Dr. L. Antognini, "Dynamic Torque Optimization Of a Step Motor by Back EMF Sensing," Proceedings of 14th Annual Symposium, Incremental Motion Control Systems and Devices, 1985, pages 293, et seq., published by Incremental Motion Control Systems Society, Champaign, Ill., 61825. This method can be used with any shape waveform. The method generally requires use of an encoder in order to produce a sufficient resolution of the rotational position of the motor shaft.